Sunday Spinelessness – Speciation by magic

By David Winter 18/09/2011

For someone that writes about evolution, I don’t spend much of my time talking about the ‘debate’ that surrounds that topic. That’s probably an artifact of living in a county that doesn’t allow people who are so confused about the world that they think the bible is a biology textbook to acquire any political power. But it’s also because debating whether evolution happened, a fact that no serious biologists has debated since Darwin’s generation and is further confirmed with each new DNA sequence, is so utterly and spectacularly boring when you compare it with some of the real debates with evolutionary biology. So here’s a little something on one debate, and the land snail shells that help swing it a little towards one side.

Some of the most contentious debates within evolutionary biology are to do with how new species arise (a process we call speciation). For instance, it’s not clear how much ecology* matters when it comes to speciation. Some authors argue that speciation and ecological adaptation are usually seperate processes – the second making species distinct only after speciation has separated them. Others argue that ecological adaptation can itself be an important part of the speciation process and maybe even be enough to drive species apart.

Like many ideas in evolution, this debate goes back to Darwin’s time. People who really ought to know better will sometimes tell you that, despite its name, The Origin of Species doesn’t have a theory of speciation. You should tell those people to read Chapter 4. Darwin did have a theory of speciation, and it explicitly placed ecological competition between newly formed species as the key to driving species apart from each other. We’ve learned a few things about biology since Darwin’s time, and it turns out his verbal arguments don’t hold up to mathematically rigorous models of the ones genes work in populations. Natural selection can’t push a population apart more quickly than genetic recombination (the mixing of genes that happens in each generation) pulls it back together. So, species can’t arise soley from selection. In fact, the modern conception of speciation revolves around the flow of genes between populations. If a population isn’t sharing genes with others it’s free to evolve independantly and take on the properties that make species distinct.

Although people have talking about gene flow with regard to speciation since Darwin’s time, Ernst Mayr is probably the person most associated with establishing this idea among evolutionary biologists. Mayr took the importance of ‘reproductive isolation’ to its logical extremes – arguing lack of gene flow was not just a pattern that created species but actually the definition of a species (I disagree) and that speciation almost exclusively occured because of geographical barriers that keep populations apart from each other (leaving no room for selection).

But the gene-flow conception of speciation still leaves a tiny bit of room for selection as a driver of speciation. For instance, imagine a trait that could, at once, be subject to ecological competition and prevent gene flow between members that don’t share the trait. Then selection would be acting to keep diverging species away form each other at the same time as adapting them to their habitat. Sergey Gavrilets, a theoretical evolutioanry biologist, called models of speciation that rely on these sort of quirks “magic trait” models, partly to represent some scepticsm that such traits could exist in the wild. But empiricists have known for a long time that these sorts of traits really are out there. For instance, many plant eating insects only mate on their host-plant. So, if two diverging species are adapting to particular hosts plants, that same adaptation process will be preventing them from mating with each other. Other examples of these magic traits include body size in fish, beak size in birds, wing colouration in butterflies and, now, shell characters in land snails.

Snails can be left- or right-handed. Or, at least, the sprial of a snail’s shell can turn clockwise (making a right-handed or dextral spiral) or anti-clockwise (a left-handed or sinistral spiral) and the direction of spiraling is decided by a single gene (inherited from the mother, suggesting in may be an imprinted gene as snail’s don’t have sex chromosomes) . Most species are predominately right-handed and very few individuals within a species don’t match the predominant spiraling direction (I only know of one exception to this rule). In fact, I’ve spent more time than most people looking at snails, and I’ve never seen a left-handed one (trust me, I check!). There’s a very good reason one individuals within one species are predominately of one spiraling direction – left-handed land snails have great trouble mating with right-handed ones. Land snails are all hermaphrodites and they mate by lining up extending their gentals through a pore on the ‘spiral side’ of their body (if you aren’t invert-phobic, there are plenty of photographs of this process here). But mirror-image snails, espacially those with relatively flat shells, struggle to line up in this way, and when they do their shells bump into each other. For this reason, ‘mirror’ snails (which do arise in populations all the time) struggle to reproduce and leave few descendants.

The direction in which a snail’s shell coils also has ecological implications. Animals that specialise in eating snails have adapted to attacking right-handed shells. So, for instance, Pareas snakes always attack from the left and have lopsided jaws that help them work the snail out of the shell:

As you might imagine, these adaptations mean the snakes are less able to attack left-handed snails. If death by snake is a big risk in a snail population, then left-handed snails, while still having a hard time when it comes to mating, will be at a distinct ecological advantage. So the direction of snail’s coil could be subject to ecological selection, and it definitely presents a potential barrier to gene flow. But to be a magic trait it needs to be doing both of these things at the same time.

The Japanese land snail genus Satsuma provides a natural experiment to test this idea. Satsuma snails come in left- and right-handed forms and some populations share their homes with the snake eating Pareas iwasakii snakes. Masaki Hoso and his colleagues (Hoso et al 2010, looked at the distribution of left- and right-handed Satsuma species and their relationships with each other.

From this data they concluded that sinistral Satsuma species have evolved multiple times and almost always in regions that are currently home to snail-eating snakes. So shell shape really does seem like a magic trait here – left handed shells get an ecological advantage that allows them to survive and it also prevents them from sharing genes with right handed snails.

So Satsuma snails are another example of magic traits in the wild. But I think they are an opportunity to understand a bit more about speciation. The hardest thing about studying speciation is separating the differences that cause speciation with those that arise once species stop sharing genes. In the case of Satsuma we know a change one gene caused speciation so any other traits that differentiate left- and right-handed snails living along side each other happened after the fact. The number of left-right species pairs, and the different ages of the lineages they represent gives us a unique chance to understand the how interactions between newly formed species shape their futures.

Surely that’s infinitely more interesting that another round of the evolution-creation controversy?

You should also check odd Ed Yong’s take on this study, which is predictably excellent.

*I’m sorry to do this, because I don’t want to be one of tiresome people who complain about the way language changes, but the science of  ecology is something quite different from what’s fast becoming the modern definition of the word. Ecology is the study of the way organisms interact with each other and their environment and (as far as I can tell) mainly involves counting a lot of things then doing some clever statistics on the resulting numbers. It’s not (directly) about conservation or sustainability and it’s certainly not an idea invented by advertisers who worked out adding ‘eco-‘ to a products name and putting it in a plain box allowed them to sell it at twice the price.